The present invention is directed to nanostructures. More particularly, the invention provides bulk nano-ribbon and/or nano-porous structures. Merely by way of example, the invention has been applied to thermoelectric devices. However, it would be recognized that the invention has a much broader range of applicability, including but not limited to use in transistor, solar power converter, battery electrodes and/or energy storage, catalysis, and/or light emitting diodes.
Conventional nanostructure devices, such as nanohole and nanomesh devices, have been shown to have good thermoelectric figures of merit ZT. ZT=S2σ/k, where S is the material's thermopower, σ is the electrical conductivity, and k is the thermal conductivity. These devices have been formed in thin silicon-on-insulator epitaxial layers or formed from arrays of nanowires, and result in nanoscale structures in thin films that are very small in physical size. For example, some conventional silicon nanoholes have been fabricated from a thin silicon film of 10-1000 nm within a conventional silicon wafer, whereby the remainder of the silicon wafer that is about 500 μm thick is etched and discarded. In another example, the resulting conventional structures are thin films and resemble ribbons, which have been shown to be microns wide and microns long, tens to hundreds of nanometers thick, with 1-100 nm diameter holes within. These conventional structures demonstrate the ability of closely-packed nanostructures to affect phonon thermal transport by reducing thermal conductivity while not affecting electrical properties greatly, thereby improving thermoelectric efficiency ZT.
Fabrication of certain nanostructures includes formation of nanowires and nanoholes from a single piece of material. For example, certain block copolymer patterning techniques are known for nano-scaled surface patterning. In another example, certain nanostructures have an aspect ratio of over 100:1 with feature size of several tens of nanometers to hundreds of nanometers. Low cost material like silicon is a target material for forming such nanostructures for the manufacture of high-performance thermoelectrics. It has been shown that silicon nanowires with low thermal conductivity can be fabricated using low cost, scalable process, but demonstrating certain difficulty in forming good electrical contacts with all nanowires. In another example, silicon nanoholes or holey silicon structures also are characterized by low thermal conductivity and being easier to form electrical contacts. But the holey silicon structures often are formed based on processes that are not very cost effective for large scale manufacture. In yet another example, the holey silicon structures are formed in silicon-on-insulator thin film which limits its scalability to form bulk-sized structures to be used for thermoelectric devices.
Hence, it is highly desirable to improve techniques of nanostructure devices.